Nanocrystal Size-Dependent Efficiency of Quantum Dot Sensitized Solar Cells in the Strongly Coupled CdSe Nanocrystals/TiO2 System

Exchange of Thiol Ligands on CuInS2 Quantum Dots in High Boiling Solvents

As-prepared quantum dots are covered with long-chain ligands to prevent aggregation. When quantum dots are used in optoelectronic devices such as solar cells and QD-LED, ligand exchange is necessary to replace long-chain ligands with short-chain ones to increase the efficiency of charge transfer from the quantum dots to the electrode. In this study, we successfully exchanged 1-dodecanethiol (DDT) ligands on CuInS2 quantum dots with mercaptopropionic acid (MPA) ligands by using a two-phase system of high-boiling hydrophilic and hydrophobic solvents. The ligand exchange to MPA was achieved by using diethylene glycol (DEG) or ethylene glycol (EG) as the hydrophilic phase and tetradecane as the hydrophobic phase. The ligand exchange rate increased with increasing ligand exchange temperature. When quantum dot sensitized solar cells (QDSSCs) were fabricated using the ligand-exchanged quantum dots, a positive correlation was observed between the progress of ligand exchange and short-circuit current density. This is because charge transfer efficiency from the quantum dots to the TiO2 electrode was improved by the ligand exchange. This work has shown that QDs synthesized using DDT can be applied to optoelectronic devices.

Electron Transfer at Quantum Dot–Metal Oxide Interfaces for Solar Energy Conversion

Electron transfer at a donor–acceptor quantum dot–metal oxide interface is a process fundamentally relevant to solar energy conversion architectures as, e.g., sensitized solar cells and solar fuels schemes. As kinetic competition at these technologically relevant interfaces largely determines device performance, this Review surveys several aspects linking electron transfer dynamics and device efficiency; this correlation is done for systems aiming for efficiencies up to and above the ∼33% efficiency limit set by Shockley and Queisser for single gap devices. Furthermore, we critically comment on common pitfalls associated with the interpretation of kinetic data obtained from current methodologies and experimental approaches, and finally, we highlight works that, to our judgment, have contributed to a better understanding of the fundamentals governing electron transfer at quantum dot–metal oxide interfaces.

Boosting Visible-Light-Driven Photocatalytic Hydrogen Production through Sensitizing TiO2 via Novel Nanoclusters

Improving the light utilization and electron-hole separation efficiency plays a central role in photocatalysis for converting light energy to hydrogen energy. Herein, for the first time, a stable, highly dispersible discrete T4 [Cd₃In₁₇Se₃₁]^5- cluster is developed as a novel photosensitizer to sensitize TiO₂ for photocatalytic hydrogen production. Compared with pristine TiO₂ (near zero) and the T4 clusters (19.5 μmol g^-1 h^-1) that exhibit low hydrogen evolution activities, the T4/TiO₂ composite, constructed from traces of 0.127 mol % T4 cluster-sensitized TiO₂, exhibits a dramatically improved photocatalytic activity of 328.2 μmol g^-1 h^-1, highlighting that the photocatalytic efficiency strongly correlates with that of the T4 cluster. In the meantime, the T4/TiO₂ composites are highly stable, remaining robust in a long-time test of 50 h for photocatalytic hydrogen production. Ultrafast transient absorption spectroscopy, in combination with electrochemical analyses, steady-state and time-resolved photoluminescence, and density functional theory calculations, indicates that the T4 cluster not only serve as a photosensitizer to absorb visible light but also form a heterojunction between the interface of the T4 cluster and TiO₂ to accelerate electron injection. This work highlights the great potential of the stable and highly dispersed discrete metal chalcogenide clusters as high-efficiency photosensitizers for converting solar energy to chemical energy.

Enhancing photocatalytic H2 evolution on In2S3/mesoporous TiO2 nanocomposites via one-pot microwave-assisted synthesis using an ionic liquid

Charge transfer and surface photocatalytic reactions are two crucial factors that determine the efficiency of photocatalysts in converting light energy to hydrogen energy. Herein, a fast and green strategy composed of a one-pot microwave-assisted solvothermal synthesis followed by a heat treatment process is developed for synthesizing In₂S₃/mesoporous TiO₂ nanocomposites, using a mixture of tetra-butyl titanate, H₂O, In-S-ionic liquid precursor solution (In-S-ILs) and [Bmim]Cl as the reactants. The ionic liquid cations act not only as a microwave absorbent during the solvothermal process, but also as a morphology-controlling agent via a dissolution-recrystallization process, leading to the formation of nanocomposites consisting of small and uniform sized In₂S₃ and mesoporous TiO₂. The optimum In₂S₃/mesoporous TiO₂ nanocomposite with an In₂S₃ content of 1.01 mol% exhibits a H₂ evolution rate of 637.9 μmol g^-1 h^-1, which is over 10 times that of commercial P25, and is 5.7 and 2.6 times that of neat mesoporous TiO₂ and the milled In₂S₃/mesoporous TiO₂ with the same component, respectively. The improved H₂ evolution activity is predominantly attributed to the small size of In₂S₃, special mesoporous structure of TiO₂, and their intimate heterojunction interface, which guaranteed effective charger transfer between In₂S₃ and mesoporous TiO₂ as well as abundant active sites for photocatalytic reactions. This work provides a novel and simple strategy for obtaining special structures of semiconductor-based nanocomposites for efficient energy conversion.

Optimal Blending of PbSe and CdSe in Polycrystalline PbCdSe Nanocomposite Film: Improved Carrier Multiplication and Enhanced Photoconversion Efficiency

The present work reports galvanostatic electro-co-deposition of n-PbCdSe semiconductor (SC) films on FTO substrate from the respective precursors. Self-designed matrices were formulated at variable concentrations of Pb²⁺ in the deposition medium. The semiconductor films constitute an intermixed structure of close-packed PbSe and CdSe nanoparticles (NPs), and the band gap (E_g) was effectively tuned in the range 0.99-1.47 eV for the variable compositions. Energy dispersive spectroscopy studies revealed that Cd exists in low level in the film matrix compared to Pb, presumably due to competitive deposition kinetics of the two chalcogenide compounds and the crystallite sizes determined from XRD studies, ranges between 15 and 12 nm, which corresponds to the size quenching of SC-NPs with increased Pb²⁺ concentration. The durability studies identify the most stable film developed at 0.025 M Pb²⁺ concentration. PbSe materials are typically characterized with impact ionization which effectively induces carrier multiplication (CM) in the quasi Type-II PbCdSe composite, exhibiting reasonably high photoconversion efficiency (PCE) of 6.14% with current output of 19.2 mA cm^-2 for the optimal PbCdSe film.

Spontaneous shape and phase control of colloidal ZnSe nanocrystals by tailoring Se precursor reactivity

A novel synthetic method of shape and phase control of ZnSe nanocrystals by tailoring Se precursor reactivity is reported.

Size-controlled electron transfer rates determine hydrogen generation efficiency in colloidal Pt-decorated CdS quantum dots

Semiconducting quantum dots (QDs) have been considered as promising building blocks of solar energy harvesting systems because of size-dependent electronic structures, e.g. QD-metal heterostructures for solar-driven H2 production. In order to design improved systems, it is crucial to understand size-dependent QD-metal interfacial electron transfer dynamics, picosecond processes in particular. Here, we report that the transfer rates of photogenerated electrons in Pt-decorated CdS QDs can be varied over more than two orders of magnitude by controlling the QD size. In small QDs (2.8 nm diameter), conduction band electrons transfer to Pt sites in an average timescale of ∼30 ps, giving a transfer rate of 2.9 × 1010 s-1 while in significantly larger particles (4.8 nm diameter) the transfer rates decrease to 1.4 × 108 s-1. We attribute this to the tuning of the electron transfer driving force via the quantum confinement-controlled energetic off-set between the involved electronic states of the QDs and the co-catalyst. The same size-dependent trend is observed in the presence of an electron acceptor in solution. With methyl viologen present, electrons leave the QDs within 1 ps for 2.8 nm QDs while for 4.6 nm QDs this process takes nearly 40 ps. The transfer rates are directly correlated with H2 generation efficiencies: faster electron transfer leads to higher H2 generation efficiencies. 2.8 nm QDs display a H2 generation quantum efficiency of 17.3%, much higher than the 11.4% for their 4.6 nm diameter counterpart. We explain these differences by the fact that slower electron transfer cannot compete as efficiently as faster electron transfer with recombination and other losses.

Fluorescence alteration of MPA capped CdSe quantum dots by spontaneous biomarker protein adsorption

Quantum dots (QDs) have significant potentials in biomedical applications of bioimaging and biosensing. Spontaneous adsorption of proteins on QDs surface is a common phenomenon, which occurred to serum proteins in biological samples, and has been observed to enhance QDs fluorescence. In this study, fluorescence alteration of 3-mercaptopropionic acid (MPA) capped CdSe quantum dots by four individual biomarker proteins was investigated. By monitoring the fluorescence emission of QDs, the biomarker protein adsorbed spontaneously on QDs surface was recognized and quantified. When alpha fetoprotein (AFP) or heat shock protein 90 alpha (HSP90α) were present, the QDs became brighter. The presence of cytochrome C (CytoC) or lysozyme (Lyz) made the QDs dimmer first, and then brighter. Within five minutes response time all four biomarker proteins were detected individually with the estimated detection limit in the range of 1-10 ng/mL and good linear dynamic ranges. The results suggested that the fluorescence of QDs was responsive to not only serum proteins but also biomarker proteins. The fluorescence response was able to correlate quantitatively with the amount of biomarker proteins in relatively low concentrations. These results provide more information to understand QDs and support their applications in biomedical fields.

Whispering Gallery Mode Enabled Efficiency Enhancement: Defect and Size Controlled CdSe Quantum Dot Sensitized Whisperonic Solar Cells

A synergetic approach of employing smooth mesoporous TiO₂ microsphere (SμS-TiO₂)–nanoparticulate TiO₂ (np-TiO₂) composite photoanode, and size and defect controlled CdSe quantum dots (QD) to achieve high efficiency (η) in a modified Grätzel solar cell, quantum dot sensitized whisperonic solar cells (QDSWSC), is reported. SμS-TiO₂ exhibits whispering gallery modes (WGM) and assists in enhancing the light scattering. SμS-TiO₂ and np-TiO₂ provide conductive path for efficient photocurrent charge transport and sensitizer loading. The sensitizer strongly couples with the WGM and significantly enhances the photon absorption to electron conversion. The efficiency of QDSWSC is shown to strongly depend on the size and defect characteristics of CdSe QD. Detailed structural, optical, microstructural and Raman spectral studies on CdSe QD suggest that surface defects are prominent for size ~2.5 nm, while the QD with size > 4.5 nm are well crystalline with lower surface defects. QDSWSC devices exhibit an increase in η from ≈0.46% to η ≈ 2.74% with increasing CdSe QD size. The reported efficiency (2.74%) is the highest compared to other CdSe based QDSSC made using TiO₂ photoanode and I⁻/I₃⁻ liquid electrolyte. The concept of using whispering gallery for enhanced scattering is very promising for sensitized whisperonic solar cells.

The role of boron in the carrier transport improvement of CdSe-sensitized B,N,F-TiO2 nanotube solar cells: a synergistic strategy

The presence of boron intensifies the synergistic effect of doping and sensitization to improve charge carrier transport into solar devices.

Double-Sided Transparent TiO2 Nanotube/ITO Electrodes for Efficient CdS/CuInS2 Quantum Dot-Sensitized Solar Cells

In this paper, to improve the power conversion efficiencies (PCEs) of quantum dot-sensitized solar cells (QDSSCs) based on CdS-sensitized TiO₂ nanotube (TNT) electrodes, two methods are employed on the basis of our previous work. First, by replacing the traditional single-sided working electrodes, double-sided transparent TNT/ITO (DTTO) electrodes are prepared to increase the loading amount of quantum dots (QDs) on the working electrodes. Second, to increase the light absorption of the CdS-sensitized DTTO electrodes and improve the efficiency of charge separation in CdS-sensitized QDSSCs, copper indium disulfide (CuInS₂) is selected to cosensitize the DTTO electrodes with CdS, which has a complementary property of light absorption with CdS. The PCEs of QDSSCs based on these prepared QD-sensitized DTTO electrodes are measured. Our experimental results show that compared to those based on the CdS/DTTO electrodes without CuInS₂, the PCEs of the QDSSCs based on CdS/CuInS₂-sensitized DTTO electrode are significantly improved, which is mainly attributed to the increased light absorption and reduced charge recombination. Under simulated one-sun illumination, the best PCE of 1.42% is achieved for the QDSSCs based on CdS(10)/CuInS₂/DTTO electrode, which is much higher than that (0.56%) of the QDSSCs based on CdS(10)/DTTO electrode.

Surface Modification of Perfect and Hydroxylated TiO2 Rutile (110) and Anatase (101) with Chromium Oxide Nanoclusters

We use first-principles density functional theory calculations to analyze the effect of chromia nanocluster modification on TiO₂ rutile (110) and anatase (101) surfaces, in which both dry/perfect and wet/hydroxylated TiO₂ surfaces are considered. We show that the adsorption of chromia nanoclusters on both surfaces is favorable and results in a reduction of the energy gap due to a valence band upshift. A simple model of the photoexcited state confirms this red shift and shows that photoexcited electrons and holes will localize on the chromia nanocluster. The oxidation states of the cations show that Ti³⁺, Cr⁴⁺, and Cr²⁺ (with no Cr⁶⁺) can be present. To probe potential reactivity, the energy of oxygen vacancy formation is shown to be significantly reduced compared to that of pure TiO₂ and chromia. Finally, we show that inclusion of water on the TiO₂ surface, to begin inclusion of environment effects, has no notable effect on the energy gap or oxygen vacancy formation. These results help us to understand earlier experimental work on chromia-modified anatase TiO₂ and demonstrate that chromia-modified TiO₂ presents an interesting composite system for photocatalysis.

Size dependence of efficiency of PbS quantum dots in NiO-based dye sensitised solar cells and mechanistic charge transfer investigation

Quantum dots (QDs) are very attractive materials for solar cells due to their high absorption coefficients, size dependence and easy tunability of their optical and electronic properties due to quantum confinement. Particularly interesting are PbS QDs owing to their broad spectral absorption until long wavelengths, their easy processability and low cost. Here, we used control of the PbS QD size to understand charge transfer processes at the interfaces of a NiO semiconductor and explain the optimal QD size in photovoltaic devices. Towards this goal, we have synthesized a series of PbS QDs with different diameters (2.8 nm to 4 nm) and investigated charge transfer dynamics by time resolved spectroscopy and their ability to act as sensitizers in nanocrystalline NiO based solar cells using the cobalt tris(4,4'-ditert-butyl-2,2'-bipyridine) complex as a redox mediator. We found that PbS QDs with an average diameter of 3.0 nm show the highest performance in terms of efficient charge transfer and light harvesting efficiency. Our study showed that hole injection from the PbS QDs to the NiO valence band (VB) is an efficient process even with low injection driving force (-0.3 eV) and occurs in 6-10 ns. Furthermore we found that direct electrolyte reduction (photoinduced electron transfer to the cobalt redox mediator) also occurs in parallel to the hole injection with a rate constant of similar magnitude (10-20 ns). In spite of its large driving force, the rate constant of the oxidative quenching of PbS by Co(iii) diminishes more steeply than hole injection on NiO when the diameter of PbS increases. This is understood as the consequence of increasing the trap states that limit electron shift. We believe that our detailed findings will advance the future design of QD sensitized photocathodes.

Quantitative Solid-State NMR Study on Ligand-Surface Interaction in Cysteine-Capped CdSe Magic-Sized Clusters

Ligand-surface interaction of semiconductor nanoparticles (NPs) controls their optoelectronic properties, and thus examination of the interaction is essential for the nanoelectronic applications of NPs. Herein, solid-state nuclear magnetic resonance (NMR) is performed to unravel the ligand-surface interaction in cysteine-capped CdSe magic-sized clusters. ¹⁵N-¹¹³Cd through-bond J-filtered NMR directly shows the presence of the nitrogen-cadmium chemical bond for the first time and indicates that ∼43% of the amines form the chemical bond. ¹⁵N-¹¹³Cd through-space dipolar-correlated NMR reveals that ∼54% of the amines locate nearby the surface cadmium with the average nitrogen-cadmium distance of 0.247 nm. The average distance is comparable with that estimated by J-filtered NMR. The difference of the two ratios (∼11%) proposes that some amines locate on the surface without forming the chemical bond, and these amines affect the relatively long observed distance in the dipolar-based experiment. Our study shows effectiveness of solid-state NMR for investigation of the ligand-surface interactions of NPs.

Inorganic Ligand Thiosulfate-Capped Quantum Dots for Efficient Quantum Dot Sensitized Solar Cells

The insulating nature of organic ligands containing long hydrocarbon tails brings forward serious limitations for presynthesized quantum dots (QDs) in photovoltaic applications. Replacing the initial organic hydrocarbon chain ligands with simple, cheap, and small inorganic ligands is regarded as an efficient strategy for improving the performance of the resulting photovoltaic devices. Herein, thiosulfate (S₂O₃^2-), and sulfide (S^2-) were employed as ligand-exchange reagents to get access to the inorganic ligand S₂O₃^2-- and S^2--capped CdSe QDs. The obtained inorganic ligand-capped QDs, together with the initial oleylamine-capped QDs, were used as light-absorbing materials in the construction of quantum dot sensitized solar cells (QDSCs). Photovoltaic results indicate that thiosulfate-capped QDs give excellent power conversion efficiency (PCE) of 6.11% under the illumination of full one sun, which is remarkably higher than those of sulfide- (3.36%) and OAm-capped QDs (0.84%) and is comparable to the state-of-the-art value based on mercaptocarboxylic acid capped QDs. Photoluminescence (PL) decay characterization demonstrates that thiosulfate-based QDSCs have a much-faster electron injection rate from QD to TiO₂ substrate in comparison with those of sulfide- and OAm-based QDSCs. Electrochemical impedance spectroscopy (EIS) results indicate that higher charge-recombination resistance between potoanode and eletrolyte interfaces were observed in the thiosulfate-based cells. To the best of our knowledge, this is the first application of thiosulfate-capped QDs in the fabrication of efficient QDSCs. This will lend a new perspective to boosting the performance of QDSCs furthermore.

A novel TiO2 nanostructure as photoanode for highly efficient CdSe quantum dot-sensitized solar cells

For sensitized solar cells, photoanodes combining the advantages of TiO₂ nanoparticles (high specific surface area) and one-dimensional (1D) nanostructures (fast transport channels) are ideal for obtaining highly efficient sensitized solar cells.

Vertically aligned ZnO/ZnTe core/shell heterostructures on an AZO substrate for improved photovoltaic performance

Vertically aligned ZnO/ZnTe core/shell heterostructures on an Al-doped ZnO substrate developed for sensitized solar cells.

Polyethylenimine-functionalized cellulose aerogel beads for efficient dynamic removal of chromium(vi) from aqueous solution

Cellulose aerogel beads with high a density of reactive amino groups were facilely prepared via a cross-linking reaction for efficient dynamic removal of Cr(vi) from aqueous solutions.